1. Field of the Invention
The present invention relates to a bias circuit for providing a bias voltage to a high-frequency amplifier. More specifically, the present invention relates to a bias circuit applicable to, for example, a so-called UWB (ultra wide band) amplifier, which requires operation in a wideband of 500 MHz or more.
2. Description of the Background Art
A conventional example of the high-frequency amplifier for use in a narrow band is disclosed in, for example, Thomas H. Lee, “The Design of CMOS Radio-Frequency Integrated Circuits,” Cambridge University Press, Cambridge, pp. 164-166 and pp. 288-292, 1998. This reference shows, on pages 164-166, an example of a cascode type amplifier for improving high frequency characteristics, and, on pages 288-292, an example of an amplifier with exemplified device constants.
A conventional example of the high-frequency amplifier for use in a wide band is taught by, for example, Paul R. Gray, et al., “Analysis and Design of Analog Integrated Circuits,” Baifukan, Japan, pp. 286-289, Oct. 20, 2000. The quality factor, which is an indicator for indicating the frequency selectivity of the high-frequency amplifier, is disclosed in, for example, Masamitsu Kawakami, “General Network Analysis I; Linear Constant (1)”, Corona Publishing Co., Ltd., Japan, pp. 72-77, 1979.
The above-indicated amplifiers present, however, the following problems. The circuit disclosed by Thomas H. Lee is apparently for a narrowband operation. It is thus difficult for the circuit to provide a wide bandwidth operation of 500 MHz or more required by the UWB amplifiers. Providing the wideband operation needs the circuit taught by Paul R. Gray, et al. The circuit cannot perform, however, a stable amplification operation at a reduced supply voltage. More specifically, it is difficult for the conventional circuits to provide both of the amplification operation with a sufficiently high gain in the wideband and the amplification operation at a lower supply voltage.